Embodiments of the present invention relate to a duct for supplying and distributing a gaseous fluid. More specifically, embodiments of the invention relate to a duct for supplying a gaseous fluid, such as to an aircraft interior, at a constant flow rate per unit of length of the duct.
In commercial cargo and commuter aircraft as well as other types of vehicles and structures, ventilation of the cargo and passenger compartments is important. These ventilation systems generally provide breathable air to clear contaminants from the interior compartments, to control humidity and temperature, and to pressurize the compartments for the health and comfort of the occupants.
Ventilation systems typically include a supply duct that extends the length of the cargo and/or passenger compartments. The air duct is generally connected to an air source and typically has one or more nozzles for distributing the air throughout the interior compartments. A typical aircraft supply duct 11 of the prior art is shown in FIG. 1, with a cross-section of the prior art supply duct 11 shown in FIG. 2.
One challenge that designers of ventilation systems have faced is non-uniform distribution of the air along the supply duct. In general, this issue is caused by a rise in the static pressure at locations within the duct that are farther away from the air source, such as near an end of the duct farthest from the air source. Longer supply ducts, such as those in longer models of aircraft, are more susceptible to this effect, known as the manifold effect.
At a location within the supply duct near the air source, the air has a high velocity as it flows through the duct, resulting in a relatively low static pressure within the duct. However, as air exits the supply duct through the nozzle outlets, the volume of air inside the duct decreases, thereby reducing the velocity of the air flow inside the duct and causing a rise in the static pressure. This rise in static pressure in turn creates a larger pressure gradient across the outlet of the nozzle, resulting in more air flow out of the nozzle than at points upstream, as shown in FIG. 1. This effect is more pronounced at points along the supply duct that are farther from the air source.
A uniform air flow rate along the supply duct is important for even distribution of air through the interior compartment. Several solutions have been proposed to address this issue, but each proposed solution has significant drawbacks.
One method of providing uniform air flow is to shorten the supply duct, thereby reducing the resulting rise in static pressure at the downstream portions of the duct. The shorter duct, however, requires more risers, or ducts that connect to the main air source, and flow balance orifices. These additional components and design considerations involve increased costs that are undesirable.
Another method that has been proposed is to reduce the size of the nozzle openings and to increase the static pressure in the duct. The disadvantage of this method is the resulting need for increased fan power to create the higher static pressure. In addition to increased costs, the higher static pressure generated within the duct causes a larger pressure gradient across the nozzle outlet that is accompanied by more noise as air flows out of the nozzle. Such an increase in noise is disturbing to occupants and is thus undesirable.
Yet another proposed solution is to taper the supply duct, such that the cross-sectional area of the duct is decreased in downstream duct sections that are farther from the air source. Although this approach may result in a reduced static pressure in downstream portions of the duct, a gradually tapered duct is rarely used due to the associated costs of manufacture and assembly. As an alternative, this method generally requires using a series of short sections of duct with steadily reducing constant diameters, resulting in a supply duct with a diameter that is gradually stepped down. This approach has its own drawbacks, however, as it involves the logistical complexity of manufacturing duct sections of various sizes, storing the different sections in inventory, and assembling the sections using the correct size of duct section at the appropriate locations and with the appropriate transitional connections.
Another method that has been proposed to produce a uniform flow rate along the duct is to use nozzles with varying orifice sizes. In downstream portions of the supply duct, farther from the air source, larger nozzle orifices would allow a greater volume of air to exit the supply duct, thereby reducing the static pressure within the duct and also reducing the velocity of the air as it flowed through the outlet. This method has practical disadvantages, however, as it once again requires non-uniform parts, which add to the cost and complexity of manufacture and construction.
Therefore, there is a need for a supply duct that provides a gaseous fluid at a constant flow rate per unit length of the duct and that uses uniform parts, is relatively simple to manufacture and assemble, and does not decrease the comfort or safety of the occupants.